1. Field of the Invention
This invention relates to a method of continuous casting of molten metal, and more particularly pertains to a continuous casting method capable of effectively suppressing formation of oscillation mark and a wrinkle, which are likely to be formed on the surface of a casting by vibration or oscillation of the mold due to application of a high frequency, at a minimum required magnetic field intensity (namely with a minimum required consumption power). Such oscillation marks and the wrinkle are likely to be formed during continuous casting while generating electromagnetic field in the mold by application of a high frequency to a mold. Hereinafter, this technique is simply referred to as xe2x80x9chigh frequency continuous castingxe2x80x9d.
2. Description of the Related Art
CAMP-ISIJ vol. 5 (1992), p200, vol. 6 (1993) p6, vol. 11 (1998), p138, and vol. 12 (1999), p53 disclose a technique of applying a high frequency to an initial solidified part of molten metal (solidification shell) which is being solidified at an initial stage of continuous casting to improve the surface properties of a resultant casting by utilizing pinching force and heating effect resulting from electromagnetic force generated by application of the high frequency. According to this technique, longitudinal slits are formed, for example, into a copper mold, and a coil is wound around the copper mold at positions corresponding to the slits (the technique applied to a cooling-type crucible) in order to quickly penetrate the electromagnetic field throughout the mold. As disclosed in Japanese Unexamined Patent Publication No. 4-178247, the width of the longitudinal slit preferably ranges from 0.2 to 0.5 mm, considering workability, permeability of magnetic field, and prevention of molten metal penetration from the mold. The total length of the slit(s) is preferably 1.5 or more times as long as the total length of the coil in terms of permeability of magnetic field.
FIG. 1 is an elevational cross sectional view showing essential parts of a generally-used casting system for use in high frequency continuous casting. In FIG. 1, numeral 1 denotes a copper mold, 2 denotes a coil for applying a high frequency, 3 denotes a slit, 4 denotes an immersion nozzle for feeding molten metal into the mold 1, F denotes flux (mold powder), ML denotes molten metal, MS denotes a solidification shell.
The system is operated in such a manner that the molten metal ML is continuously fed into the mold 1 through the immersion nozzle 4 while acting an electromagnetic force to the initial solidified part of the molten metal ML, namely, the solidification shell MS through a magnetic field which is generated by energizing the coil 2. Pinching force on the initial solidified molten metal is activated by the electromagnetic force along with the heating effect on the mold, while a casting which has been molded from the solidification shell MS is continuously or intermittently withdrawn downwardly from the system.
The flux F is loaded on the top portion of the molten metal ML inside the mold 1. The flux F serves to prevent heat radiation and to prevent oxidation of the molten metal ML. The flux F is flow into a gap between the solidification shell MS and the mold 1 to make the contact surface therebetween smooth. Thus, the flux F also serves to improve the surface properties of the resultant casting.
There has been known a phenomenon that oscillation mark is likely to be formed on the surface of the casting due to up and down oscillation of the mold during the continuous casting. Oscillation mark, when the depth thereof is great, likely causes a crack in the resultant casting. Also, there has been known that inclusions and bubbles are likely to be entrapped in a so-called xe2x80x9chookxe2x80x9d (a discontinuously solidified part of the casting which is likely to be formed underneath the outer surface of the casting) thereby causing defect in the casting. In view thereof, it is significantly important to find a technique of suppressing oscillation mark formation in order to produce a defect-free casting with good surface properties.
After intensive study of the high frequency continuous casting method of steel, the inventors of this invention accomplished and proposed the technique disclosed in Japanese Unexamined Patent Publication No. 7-1093. The publication discloses a technique of improving the surface properties of castings while suppressing the formation of oscillation marks on the surface of castings. Particularly, the disclosed technique is a technique of properly controlling an electromagnetic field intensity or a magnitude of an electromagnetic field (in other words, magnetic flux density) of a core or hollow portion of the mold depending on the casting velocity in order to stabilize a meniscus portion of the molten metal in the mold or molten bath. According to this technique, the quantity of flux (mold powder) supplied into a gap between the initial solidification shell MS and the mold 1 is properly controlled without causing excessive internal flow in the molten bath. Employing this technique enables to raise the casting velocity to a certain level while suppressing deterioration of the surface properties of the casting.
In addition, the aforementioned technique is advantageous in the following aspects.
(i) Pinching force generated by a magnetic field enlarges the gap for the flux inflow between the initial solidification shell and the mold, thereby improving contact surface smoothness between the mold and the resultant casting. Consequently, stabilized high speed casting is secured while suppressing formation of oscillation mark.
(ii) The pinching force on the initial solidification shell brings the resultant casting into gentle contact with the mold. This is effective in suppressing adverse influence to the casting which is likely to be caused by oscillation of the mold, thereby contributing to suppression of oscillation mark formation to some extent.
(iii) The surface of the molten metal in the mold is heated up by the electromagnetic force during application of a high frequency. Spontaneously, the heat generated by the electromagnetic force starts solidifying the molten metal from the top surface thereof. This is effective in suppressing fluctuation of the molten metal surface, namely, a meniscus portion of the molten metal which may adversely affect formation of a solidification shell, thereby contributing to improvement of surface quality of the casting.
(iv) The combination of heating and pinching force enables to prevent the solidification shell from protruding above the top surface of the molten metal. This arrangement is effective in preventing entrapment of gas bubbles and inclusions in the solidification shell, thereby contributing to improvement of the properties underneath the outer surface of the casting.
The above technique is advantageous in various ways as mentioned above because the technique considers controllability of magnetic field intensity (magnetic flux density) in the core or hollow portion of the mold in such a manner that formation of oscillation mark is suppressed even under a condition where a deep oscillation mark is liable to be formed. However, the required field intensity varies depending on oscillation conditions of the mold, the publication does not give full consideration to field intensity required under this conditions to suppress formation of oscillation mark.
CAMP-ISIJ vol. 12 (1999), p57 reports an experiment concerning continuous molding of steel with use of a high frequency of 20 kHz. The publication reports that the experiment improved depth of oscillation mark from 0.6 mm to 0.2 mm. This report, however, is silent about an optimum field condition that enabled depth of oscillation mark to such a small value. Also, the experiment was performed under a single oscillation condition. The publication, accordingly, does not provide technical data relating to correlation between mold oscillation which affects depth of oscillation mark and magnetic field intensity required to suppress formation of oscillation mark.
It should be noted that depth of oscillation mark and negative time strip tn have a close correlation. The shorter the negative time strip tn is, the smaller the depth of oscillation mark is. Shortening the negative time strip tn, however, resultantly increases the number of oscillation of the mold per unit time. The increased number of oscillation undesirably likely to form oscillation mark in a casting. Thus, there is room for developing the technique of completely eliminating formation of oscillation mark.
In the case where a mold is not oscillated or continuous casting is conducted in such a condition as to accomplish the negative time strip tn to 0 or less, there has been empirically known that a wrinkle, such as irregularity or disorder, is generated on the surface of a casting. Therefore, there is also room for elucidating magnetic field intensity required to securely prevent occurrence of such a defect.
It is an object of the present invention to provide a continuous casting method which has overcome the afore-mentioned problems residing in the prior art.
According to an aspect of this invention, a method of continuously casting molten metal comprises the step of feeding molten metal into a mold to produce a casting continuously while generating an electromagnetic field in the mold by applying a high frequency to the mold. The application of the high frequency is controlled in such a manner that a magnitude of an electromagnetic field which is applied to a solidification shell forming start location of the mold from which a solidification shell of the casting starts to be formed becomes equal to or greater than a minimum required flux density to be applied to the mold. The minimum required flux density is determined based on a negative time strip and a frequency in the electromagnetic field as operation parameters according to the following equation:
Bmin=1130xc3x97tnxe2x88x925fxc3x97(tnxe2x88x920.05) 
where
tn=cosxe2x88x921 (v/2xcfx80xc3x97fmxc3x97a)/(xcfx80xc3x97fm)
Bmin: minimum required flux density (gauss)
tn: negative time strip (second)
f: frequency in electromagnetic field (kHz)
v: casting velocity (m/sec)
fm: number of oscillation or frequency of mold (Hz)
a: one-way stroke of mold (m).